There is a pressing need for developing ultra-small scanning optical probes. These probes require ultra-small imaging lenses and associated scanning and beam director elements. Such probes are used in Optical Coherence Tomography (OCT) and other interferometric imaging and ranging systems, as well as for delivery of other imaging modalities (e.g. fluorescence) or therapeutic optical sources. Future medical (and nonmedical) optical probes will require these miniature probes to navigate small and torturous passageways such as arteries, veins, and pulmonary airways. Present technology generally is not adequate for meeting the needs of these small probes when the probes must be less than ˜500 μm in diameter, while simultaneously having a working distance that can extend up to several millimeters and performing controlled and potentially complex scan patterns.
Although the design and construction of small lenses is known, as exemplified by a design of a catheter that uses a small (˜1 mm) GRIN lens coupled to a fold mirror for imaging the aperture of a single-mode fiber onto a vessel wall, the scaling of this design to less than 500 μm is problematic. Although techniques exist for making very small lenses that have small working distances suitable for coupling to laser diodes and other optical components, these lenses generally do not offer the >1 mm working distance and the >1 mm depth-of-field required for many applications.
Further, there are a number of commercially available ‘torque wires’—miniature wire-wound devices intended to transmit torque over a long and flexible shaft. Such devices are now commonly used in intravascular ultrasound (IVUS) procedures. Such ultrasound probes combined with torque wires perform rotational scanning in coronary arteries. Generally however, these devices are at least 1 mm in diameter, and are thus 2 to 4 times larger than the devices required by many applications. Presently, such torque wires are not scalable to the sizes required to permit the construction of small optical scanning probes.
U.S. Pat. No. 6,165,127 ('127) discloses the use of a viscous fluid located inside the bore of an ultrasound catheter. The purpose of the fluid is to provide loading of a torque wire such that the wire enters the regime of high torsional stiffness at moderate spin rates. As described in the '127 patent, this fluid is housed within a separate bore formed inside the main catheter, increasing the overall size of the device, the fluid does not contact the imaging tip, nor does the ultrasound energy propagate through this fluid unlike the present invention.
Finally, achieving uniform rotational scanning at the distal tip of a single fiber, while maintaining an overall device size less than 500 um in diameter is a major challenge. Because it is highly undesirable to add a motor to the distal tip, with the attendant wires and size issues, a way must be found to apply torque to the proximal tip and transmit the torque to the distal tip which may be as much as three meters away in a catheter application. If the extremely low inherent rotational stiffness of a glass fiber is considered (approximately 1 millionth of a N-m of applied torque will cause a 1 cm length of standard 125 μm diameter fiber to twist up one degree) the issues of uniformly spinning the distal tip by driving the proximal end can be appreciated. Uniform rotation is critically important in endoscopic techniques in order to obtain accurate circumferential images. The term ‘NURD’ (non-uniform rotational distortion) has been coined in the industry to describe these deleterious effects.
The present invention relates to a small optical fiber probe that experiences substantially no NURD.